Possible options for implicit casting

[certik]

Now when #1271 is in, I suggest we keep it for some time and see how it feels.

If we decide we want to implement some implicit casting, it seems we can't do any implicit casting in assignment, because in CPython the LHS type is taken from the RHS, so LPython must follow the same approach in order to be compatible, so we cannot do any implicit casting there.

What we can consider doing is:

x: f32
x = 5.5
x = x * 5
x = x / 5

Where we can do implicit casting from integer to real. That casting I think is always safe and it would cast to the floating point type of the other operand in the binary operation. I think it is natural to write things like 2*a*b for floating point a, b.